Furnace having fingers interdigitatedly engaged with its heating elements



Oct. 15, 1968 0. G. PEDROTTI 3,406,275

FURNACE HAVING FINGERS INTERDIGITATEDLY ENGAGED WITH ITS HEATING ELEMENTS Filed Dec. 2, 1965 I NVENTOR.

I DONALD c. PEDROTTI BY ATTORNEY United States Patent 3,406,275 FURNACE HAVING FINGERS INTERDIGI- TATEDLY ENGAGED WITH ITS HEAT- ING ELEMENTS Donald G. Pedrotti, Cupertino, Calif assignor to 'RCK, Incorporated, Sunnyvale, Calif. Filed Dec. 2, 1965, Ser. No. 511,086 4 Claims. (Cl. 219-406) ABSTRACT OF THE DISCLOSURE A furnace for heating objects at high temperatures in a controlled-atmosphere having a heating unit surrounding the objects to be heated, the heating unit having a support with interdigitated fingers engaging the heating elements, the fingers extending inwardly towards the interior of the heating unit at least beyond the heating elements, the tips of the fingers being adapted to support and come into intimate contact with the tube fitting into the interior of the heating unit, and an insulating layer surrounding the support to prevent the passage of heat from the heating unit to the atmosphere.

The subject invention relates to a furnace adapted for heating objects at high temperatures in a controlled atmosphere. More particularly, the invention is directed to a new type of furnace construction which provides for more effective and more efficient use of heat than the furnaces of the prior art.

Although it has other purposes, the furnace of this invention is particularly adapted for use in the diffusion and oxidation of semiconductor material. In this process, wafers of semiconductor material, about the size of a half dollar, are stacked in a quartz boat and inserted into the furnace. The interior contains a cylindrical tube, generally fabricated from quartz. Surrounding the quartz tube is a heating unit and a support to hold the heating unit. One of the principal difliculties with the furnances of the prior art is the inability of the heating unit to properly support the quartz tube. Accordingly, the tube tends to sag at high temperatures, destroying its uniform shape and shortening its life. Such an effect also tends toward nonuniform heating of the wafers inside the quartz tube.

Another difiiculty encountered in the prior art furnaces is their inability to withstand high temperatures for long periods of time. Commonly the supporting member for the heating element becomes cracked and has to be re placed at frequent intervals.

Still a third problem arises in obtaining insulation which will withstand the high temperatures for long periods of time, and yet provide a sufiicient degree of insulation so that the furnace temperature may be maintained at a uniform, high level.

The furnace of this invention overcomes all of the above problems of the prior art. Briefly, the furnace of this invention adapted for heating objects at high temperatures in a controlled atmosphere comprises: a heating unit adapted to substantially surround the objects to be heated, the unit having a periphery of resistance heating elements; a support means for the heating unit having fingers interdigitatedly engaged with the heating elements and extending inwardly towards the interior of the heating unit at least beyond the element, the tips of the fingers being adapted to support an enclosure fitting into the interior of the heating unit; and an insulating means surrounding the support means adapted to prevent the passage of heat from the heating unit to the atmosphere. These fingers extending beyond the heating element towards the interior of the furnace are used to support the quartz tube. These 3,406,275 Patented Oct. 15, 1968 fingers prevent the tube from sagging, thereby increasing its life and effectiveness.

The insulating layer surrounding the heating unit is a ceramic fiber adapted to prevent the passage of heat from the heating unit to the atmosphere. This fiber combines the advantages of easy fabrication, long life, and excellent insulating properties. It has been found far superior to the solid insulating materials, such as insulating brick, commonly employed in the prior art.

The invention will be understood in more detail from the following description, making reference to the drawings in which:

FIG. 1 is a perspective view of the completed furnace of'this invention, omitting the conventional stand normally used to support the furnance;

FIG. 2 is a lengthwise cross section of the furnace taken along the plane 2-2 shown in FIG. 1;

FIG. 3 is a detailed assembly view showing the pieces making up the support means and the fingers; and

FIG. 4 is a partial cross section taken along the plane 44 shown in FIG. 2.

Referring now to FIG. 1, the completed furnace is shown having a metal, e.g., aluminum, outer cylindrical wall 10 and end plates 9, only one of which shows in FIG. 1. These plates are formed in two segments, as shown. This wall is fabricated in two sections 11 and 12 connected together at diametrically opposed points on the cylinder by conventional fasteners 13. End brick 14 extends from the end of the furnace. This end brick serves to seal the heating element and its supporting member inside the furnace. The details of this insulating material will be described later. Finally the tube 16 which supports the objects being heated extends from end brick 14, as shown. Commonly, this tube is quartz. The tube itself is not a part of this invention, and in fact is not always furnished with the furnace as sold. Often the customer supplies his own quartz tube to fit his particular applica tion, as the specific properties of the quartz tube can be varied to suit the temperatures, gases, and materials being employed in the processing operation.

Referring now to FIG. 2, heating unit 20 is made up of a plurality of heating elements 21 which together form the helical unit 20. Heating unit 20 is preferably an ironcobalt-chromium-aluminum alloy sold in the trade under the name Kanthal by the Kanthal Corporation. As shown, in this embodiment of the invention, the heating unit is unitary, all of the heating elements 21 being a part of one single helical unit 20. This is not necessary; some furnaces of this type employ more than one separate heating section. However, whether the separate sections are separate or unitary, they may be controlled separately. Note in this embodiment there is an electrical connection 22 to one portion of heating unit 20, and another connection 23 to another portion. These separate connections are used to separately control portions of unit 20 to achieve different temperatures at diiferent portions of the heating element. Accordingly, it is possible to direct more heat to the end segment of element 20 between connections 22 and 23 than the central segment. This automatically compensates for losses of heat into the ambient atmosphere outside at the ends of the furnace. Connections 24 and 25 at the other end serve the same function. Note, however, that the distance between the points where connections 24 and 25 are connected to element 20 is shorter than the distance between the points where connections 22 and 23 are connected to element 20. It has been found that using a shorter distance between the connections at the point where the material to be heated enters into the furnace and a longer distance between connections at the other end where the gases used in the process are normally introduced results in a longer portion of the furnace having a uniform or flat temperature zone. The longer the fiat zone, the larger the capacity of the furnace for objects to be uniformly heated, such as semiconductor wafers.

The support means 26 for heating unit 20 has two parts. The first part is a plurality of fingers 27 interdigitatedly engaged'with the periphery of heating elements 21 and extending inwardly towards the interior of heating unit 20 at least beyond elements 2-1 themselves, as best shown in FIG. 4. To accomplish this, the over-all length of fingers 27 must exceed the cross-sectional diameter of elements 21, as shown in FIGS. 2 and 4. This permits the ends of fingers 27 to extend into the interior of heating unit 20. The tips of these fingers are adapted to support an enclosure, such as quartz tube 16 shown in FIG. 1, which in turn fits into the interior of the heating unit as shown in FIG. 1. The second major component of the support 26 for heating unit 20 is the insulating shell surrounding the heating unit and interposed between the heating unit and the insulating layer 15. This shell is composed of small interlocked sections 29 no one section making up more than one-tenth, preferably one-twentieth, of the surface area of the entire shell. In the preferred emobdirnent, 80 sections were employed. The shell itself is made of a ceramic material which is capable of withstanding heat. Such material is sold, for example, by the Electrical Refractories Company. The particular advantage of this invention in using small interlocked sections is that the shell is then capable of withstanding very wide temperature variations and their concomitant expansion and contraction without danger of cracking. The interlocking of the sections, as shown in more detail in FIG. 3, provides excellent insulation; the fact that they are sections rather than integral does not seriously detract from their insulating ability. Accordingly, the furnace is useful over a wider range of temperatures than furnaces of the prior art.

As shown in FIG. 3, the ceramic pieces making up fingers 27 are interdigitated with the ceramic shell components 29, which together make up support 26 of this invention to support heating unit 20.

Referring to FIGS. 2 and 4, insulating material forms an important part of the invention. This material is a soft, formable, heat-resistant insulating material which surrounds heating unit and support 26. Preferably this insulating material is a ceramic fiber. Examples are potassium-titanate fibers, alumina-silicate fibers, and the like. One type of alumina-silicate fiber is sold under the trade name Kaowool by the Babcock and Wilcox Corporation; another type called Fiberfrax is sold by the Carborundum Corporation. This material has all of the advantages of being a good insulator, but is soft and formable. It is therefore capable 'of being packed tightly into rim 11. The insulating material itself, as shown in FIG. 4, is surrounded by layer 30, which is normally the same material as the insulating material 15 but in hardened form. The hardened layer 30 is normally about one half inch thick, and is formed by molding. The layer is formed in four quarters and assembled into shell halves 11 and 12, as shown in FIG. 4. Heating unit 20' and support 26 are placed inside segment 12, as shown. The other segment 11 is placed on top, and the two segments are then fastened as shown in FIG. 1. Insulating layer 15 is packed tightly inside. One type of material used for the hardened insulating layer 39 is called WRP Wrap, and is also available from Babcock and Wilcox 5 Corporation.

The above detailed description obviously represents only a preferred embodiment of'the invention. It will be obvious to one skilled in the art that many modifications, additions, and alterations may be made in this preferred embodiment without departing from the true spirit and scope of the invention. Accordingly the only limitations to be placed on the scope of this invention are those set forth in the claims which follow:

What is claimed is:

1. A furnace adapted for heating objects at high temperatures in a controlledatmosphere, comprising:

a heating unit adapted to substantially surround the objects to be heated, said unit having aperiphery of resistance heating elements, i 1, 20 a support means for said heating unit having fingers interdigitatedly engaged with said heating elements, and extending inwardly towards the interior of said heating unit at least beyond said elements, the tips of said fingers being adapted to support and come into intimate contact with a tube fitting into the interior of said heating unit; and

an insulating means surrounding said support means adapted to prevent the passage of heat from said heating unit to the atmosphere.

2. The furnace of claim 1 further characterized by said shell being composed of interlocked sections, no one section making up more than one-tenth of the surface area of said shell.

3. The furnace of claim 2 further characterized by sai sections being a ceramic material. I

4. A furnace adapted for heating objects at high temperatures in a controlled atmosphere, comprising:

a heating unit adapted to substantially surround the object to be heated, said unit having a helical resistance heating element;

support means for said element having fingers interdigitating said helical element and extending inwardly towards the center of the helix beyond the thickness of the element, the tips of said fingers being adapted to support and come into intimate contact with a tube fitting in the interior of said helix;

an insulating layer of a ceramic fiber surrounding said heating unit and support means adapted to prevent the passage of heat from, said heating unit to the atmosphere.

References Cited UNITED STATES PATENTS 1,678,875 7/1928 Rohn 13-22 1,959,358 5/1934 Gouverneur 219-406 2,254,047 8/1941 Roth 13-20 2,823,292 2/1958 Kunzle 219388 3,361,863 1/1968 Lang 219-390 BERNARD A. GILHEANY, Primary Examiner.

H. B-ROO'ME, Assistant Examiner. 

